Implement fract, frexp and modf for half

http://b/23536224

Implementations are based on float32 implementations in
bionic/libm/upstream-bsd.

(cherry picked from commit b32dc1237e7e158bab7d15ab18618c4916e64415)

Change-Id: Ibf927cbab3aa45c7bedbc5ec23c6f31eafe92bd9
2 files changed